Staged combustion properties for pulverized coals at high temperature

Staged combustion properties for pulverized coals have been investigated by using a new-concept drop-tube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. Reaction temperature (1800–2100 K) and time (1–2 s) were similar to those used in actual boilers. When coal was burnt at the same stoichiometric ratio as in actual boilers, similar combustion performance values as for actual boilers were obtained regarding NO_x emission and carbon in ash. The most important factor for low NO_x combustion was to raise the combustion temperature above the present range (1800–2100 K) in the fuel-rich zone. The NO_x emission was significantly increased with decrease of burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. But, NO_x emission was cut to around 100–150 ppm, for sub-bituminous coal and hv-bituminous coal, in the latest commercial plants by forming this high-temperature fuel-rich region in the boilers. If the temperature and stoichiometric ratio could be set to the most suitable conditions, and, burning gas and air were mixed well, it would be possible to lower NO_x emission to 30–60 ppm (6% O₂). The most important NO_x reduction reaction in the fuel-rich zone was the NO_x reduction by hydrocarbons. The hydrocarbon formation rate in the flame was varied with coal properties and combustion conditions. The NO_x was easily reduced when coals which easily formed hydrocarbons were used, or, when burning conditions which easily formed hydrocarbons were chosen. Effects of burning temperature and stoichiometric ratio on NO_x emission were reproduced by the previously proposed reaction model. When solid fuel was used, plant performance values varied with fuel properties. The proposed drop-tube furnace system was also found to be a useful analysis technique to evaluate the difference in combustion performance due to the fuel properties.     

The effect of low-NOx combustion on residual carbon in fly ash and its adsorption capacity for air entrainment admixtures in concrete

Fly ash from pulverized coal combustion contains residual carbon that can adsorb the air-entraining admixtures (AEAs) added to control the air entrainment in concrete. This is a problem that has increased by the implementation of low-NO_x combustion technologies. In this work, pulverized fuel has been combusted in an entrained flow reactor to test the impact of changes in operating conditions and fuel type on the AEA adsorption of ash and NO_x formation.Increased oxidizing conditions, obtained by improved fuel-air mixing or higher excess air, decreased the AEA requirements of the produced ash by up to a factor of 25. This was due to a lower carbon content in the ash and a lower specific AEA adsorptivity of the carbon. The latter was suggested to be caused by changes in the adsorption properties of the unburned char and a decreased formation of soot, which was found to have a large AEA adsorption capacity based on measurements on a carbon black. The NO_x formation increased by up to three times with more oxidizing conditions and thus, there was a trade-off between the AEA requirements of the ash and NO_x formation. The type of fuel had high impact on the AEA adsorption behavior of the ash. Ashes produced from a Columbian and a Polish coal showed similar AEA requirements, but the specific AEA adsorptivity of the carbon in the Columbian coal ash was up to six times higher. The AEA requirements of a South African coal ash was unaffected by the applied operating conditions and showed up to 12 times higher AEA adsorption compared to the two other coal ashes. This may be caused by larger particles formed by agglomeration of the primary coal particles in the feeding phase or during the combustion process, which gave rise to increased formation of soot.     

The potential for low NOx from a precessing jet burner of coal

The effectiveness of the Precessing Jet nozzle at yielding low NO_x levels from burning coal was examined at the pilot scale. Thus, a coal burner of nominal thermal load of 138 kW was sampled two dimensionally, and subsequently modeled. The one-dimensional, steady-state, semiempirical mathematical model considered the release and combustion of volatiles, and subsequent oxidation of char. A comprehensive reaction scheme was formulated to account for the oxidation of the resulting CO, the formation of NO from various sources (volatile, char, preheated air) and the oxidation of H₂S to SO₂. The agreement between the experimental and predicted profiles of coal burnout, [O₂], [NO] and gas temperature was not good near the burner; however, the agreement improved in the postflame region. The model was also used to simulate the center line characteristics of the same flame, but with the secondary air preheated to 500°C. This flame was then scaled for constant velocity and constant residence time to 20 MW. It was deduced that constant residence-time scaling predicts ignition and combustion of the coal at the same axial location as the baseline flame. However, constant velocity scaling shifts combustion closer to the burner. Constant-velocity scaling was found to be more suitable for the theoretical scaling of pulverized coal flames. It was not possible to comment on the potential of the burner for low NO_x in cement kilns, because the measured and computed gas temperatures were low. However, the model predicted low concentrations of fuel NO_x.     

Numerical simulation on pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed

High temperature air combustion is a prospecting technology in energy saving and pollutants reduction. Numerical simulation on pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed was presented. The down-fired combustor, taken as the calculation domain, has the diameter of 220 mm and the height of 3000 mm. 2 cases with air staging combustion are simulated. Compared the simulation results with experimental data, there is a good agreement. It is found that the combustion model and NOx formation model are applicable to simulate the pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed. The results show that there is a uniform temperature profile along the axis of the down-fired combustor. The NOx emissions are lower than those of ordinary pulverized coal combustion, and the NOx emissions are 390 mg/m3 and 352 mg/m3 in Case 1 and Case 2, respectively. At the range of 300-600 mm below the nozzle, the NO concentration decreases, mainly resulting from some homogeneous reactions and heterogeneous reaction. NO concentration has a little increase at the position of 800 mm below the nozzle as the tertiary air supplied to the combustor at the position of 600 mm below the nozzle.     

A role of hydrocarbon reaction for NOx formation and reduction in fuel-rich pulverized coal combustion

We have investigated an index for modeling a NO_x reaction mechanism of pulverized coal combustion. The reaction mechanism of coal nitrogen was examined by drop-tube furnace experiments under various burning conditions. We proposed the gas phase stoichiometric ratio (SRgas) as a key index to evaluate NO_x concentration in fuel-rich flames. The SRgas was defined as:

     

Experimental study on combustion of torrefied palm kernel shell (PKS) in oxy‐fuel environment

Oxy‐combustion of biomass can be a major candidate to achieve negative emission of CO₂ from a pulverized fuel (pf)‐firing power generation plants. Understanding combustion behavior of biomass fuels in oxy‐firing conditions is a key for design of oxy‐combustion retrofit of pulverized fuel power plant. This study aims to investigate a lab‐scale combustion behavior of torrefied palm kernel shell (PKS) in oxy‐combustion environments in comparison with the reference bituminous coal. A 20 kW_th‐scale, down‐firing furnace was used to conduct the experiments using both air (conventional) and O₂/CO₂ (30 vol% for O₂) as an oxidant. A bituminous coal (Sebuku coal) was also combusted in both air‐ and oxy‐firing condition with the same conditions of oxidizers and thermal heat inputs. Distributions of gas temperature, unburned carbon, and NO_x concentration were measured through sampling of gases and particles along axial directions. Moreover, the concentrations of SO_x and HCl were measured at the exit of the furnace. Experimental results showed that burnout rate was enhanced during oxy‐fuel combustion. The unburnt carbon in the flue gas was reduced considerably (~75%) during combustion of torrefied PKS in oxy‐fuel environment as compared with air‐firing condition. In addition, NO emission was reduced by 16.5% during combustion of PKS in oxy‐fuel environment as compared with air‐firing condition.     

Promising components of waste-derived slurry fuels

The paper presents the results of experimental studies of energy (calorific value, ignition delay times and threshold ignition temperatures, duration and temperature of combustion) and environmental (CO₂, NO_x and SO_x emission) characteristics of fuel slurries based on pulverized wood (sawdust), agricultural (straw), and household (cardboard) waste. Wastewater from a sewage treatment plant served as a liquid medium for fuels. Petrochemical waste and heavy oil were additives to slurries. The focus is on obtaining the maximum efficiency ratio of slurry fuel, calculated taking into account environmental, cost, energy and fire safety parameters. All slurry fuels were compared with typical coal-water slurry for all the parameters studied. A comparison was also made between slurries and traditional boiler fuels (coal, fuel oil). The relative efficiency indicator for waste-based mixtures was varied in the range of 0.93–10.92. The lowest ignition costs can be expected when burning a mixture based on straw, cardboard and oil additive (ignition temperature is about 330 °C). The volumes of potential energy generated with the active involvement of industrial waste instead of traditional coal and oil combustion are forecasted. It is predicted that with the widespread use of waste-derived slurries, about 43% of coal and oil can be saved.     

A numerical study of pulverized coal ignition by means of plasma torches in air–coal dust mixture ducts of utility boiler furnaces

Paper presents selected results of numerical simulation of processes in air–coal dust mixture duct of pulverized coal utility boiler furnace with plasma-system for pulverized coal ignition and combustion stabilization. Application of the system in utility boiler furnaces promises to achieve important savings compared with the use of heavy oil burners. Plasma torches are built in air–coal dust mixture ducts between coal mills and burners. Calculations have been performed for one of rectangular air–coal dust mixture ducts with two opposite plasma torches, used in 210 MW_e utility boiler firing pulverized Serbian lignite. The simulations are based on a three-dimensional mathematical model of mass, momentum and heat transfer in reacting turbulent gas-particle flow, specially developed for the purpose. Characteristics of processes in the duct are analyzed in the paper, with respect to the numerical results. The plasma-system thermal effect is discussed as well, regarding corresponding savings of liquid fuel. It has been emphasized that numerical simulation of the processes can be applied in optimization of pulverized coal ignition and combustion stabilization and enables efficient and cost-effective scaling-up procedure from laboratory to industrial scale.     

A numerical simulation of pulverized coal combustion employing a tabulated-devolatilization-process model (TDP model)

A new coal devolatilization model employing a tabulated-devolatilization-process model (TDP model) is developed, and its validity is investigated by performing a numerical simulation of a pulverized coal combustion field formed by an industrial low-NO_x burner in a 100 kg-coal/h test furnace. The predicted characteristics of the pulverized coal combustion field obtained from the simulation employing the TDP model are compared with those employing the conventional devolatilization model, those employing the two competing reaction rate model, and the experiments. The results show that drastic differences in the gas flow patterns and coal particle behavior appear between simulations. In particular, the recirculation flow behavior is strongly affected by the difference in the coal devolatilization model because of the difference in the volatile matter evolution rate. The TDP model captures the observed behavior of the coal particles in the experiment better than the other models. Although it is considered that by adjusting the devolatilization parameters the prediction similar to the TDP model is also possible by the other models, appropriate devolatilization parameters are automatically set to particles depending on the particle heating rate without trial–error method by employing the TDP model.     

Study on the NOx release rule along the boiler during pulverized coal combustion

Numerical simulation and experimental study on NO _x release along the boiler during pulverized coal combustion have been conducted. With the increase of temperature the NO _x emission increased and the peak value of NO _x release moved forward. But when the temperature increased to a certain degree, NO _x emission began to reduce. NO _x emission increased with the increase of nitrogen content of coal. The peak value of NO _x release moved backwards with the increase of coal rank. NO _x emission increased obviously with the increase of stoichiometric ratio. There existed a critical average diameter of the pulverized coal (d _c ). If d ⩽ d _c , NO _x emission reduced with the decrease of pulverized coal size. If d > d _c , NO _x emission reduced with the increase of the pulverized coal size. The results showed that the simulation results are in agreement with the experimental results for concentration distribution of NO _x along the axis of the furnace. Translated from Proceeding of the CSEE, 2006, 26(1): 35–39 [译自: 中国电机工程学报]     

Multi‐objective optimization of the coal combustion performance with artificial neural networks and genetic algorithms

The present work introduces an approach to predict the nitrogen oxides (NO_x) emissions and carbon burnout characteristics of a large capacity pulverized coal‐fired boiler with an artificial neural network (ANN). The NO_x emissions and carbon burnout characteristics are investigated by parametric field experiments. The effects of over‐fire‐air (OFA) flow rates, coal properties, boiler load, air distribution scheme and nozzle tilt are studied. An ANN is used to model the NO_x emissions characteristics and the carbon burnout characteristics. A genetic algorithm (GA) is employed to perform a multi‐objective search to determine the optimum solution of the ANN model, finding the optimal setpoints, which can suggest operators' correct actions to decrease NO_x emissions and the carbon content in the flyash simultaneously, namely, get a good boiler combustion performance with high boiler efficiency while keeping the NO_x emission concentration meet the requirement. Copyright © 2005 John Wiley & Sons, Ltd.     

A dimensionless factor characterizing the ignition of pulverized coal flow: Analytical model,experimental verification,and application

An analytical model describing the ignition process of pulverized coal is proposed, and a dimensionless condition number (N_com) is obtained to describe the comprehensive effect of factors governing the ignition of pulverized coal flow, such as the initial temperature of flow, the sectional heat load of the furnace, and the flux of primary air, secondary air and recirculation flue gas. An optimized concentration of pulverized coal flow is derived explicitly, upon which the earliest ignition of pulverized coal flow is possible. The model is verified in a hot furnace experiment, where it is shown that the derived criterion (N_com) can be used for different kinds of coal and different types of burner. For given coal and sectional heat load of furnace, when the value of N_com increases, the condition of ignition is improved and both unburned carbon and NO_x emission are reduced. The employment of N_com in the optimization of burner operating conditions is demonstrated through two applications. In practice, the criterion N_com can be used to guide the selection of the concentration and type of pulverized coal, as well as the choice of burner and desired aerodynamic field, so as to achieve an optimized performance. Copyright © 2008 John Wiley & Sons, Ltd.     

旋风炉内气相燃烧及两相流动的数值模拟

在有反应两相流动及煤粉燃烧的全双流体模型（ＰＴＦ模型,ｐｕｒｅ ｔｗｏ－ｆｌｕｉｄ ｍｏｄｅｌ）基础上,采用修正的ｋ－ε－ｋｐ两相湍流模型,对旋风炉内的湍流气相燃烧（甲烷和一氧化碳的燃烧）及在气相燃烧条件下的两相流动进行了数值模拟研究,模拟结果表明,在有燃烧的情况下,在旋风炉的底部存在近壁回流区,该回流区有利于火焰稳定,气粒两相切向速度分布具有类似的Ｒａｎｋｉｎｅ涡结构,该研究为煤粉燃烧的数值模拟     

Modeling of NOx formation in diesel engines using finite-rate chemical kinetics

A fast, physics-based model to predict the temporal evolution of NO_x in diesel engines is investigated using finite-rate chemical kinetics. The temporal variation of temperature required for the computation of the reaction rate constants is obtained from the solution of the energy equation. NO_x formation is modeled by using a six step mechanism with eight species instead of the traditional equilibrium calculations based on the Zeldovich mechanism. Fuel combustion chemistry is modeled by a single-step global reaction. Effects of various stages of combustion on NO_x formation is included using a phenomenological burning rate model. The effects of composition and temperature on the thermophysical properties of the working fluid are included in the computations. Comparison with measured single-cylinder engine-out NO shows good agreement with experimental data. The validated model is then used to demonstrate the impact of various operating parameters such as injection timing and EGR on engine-out NO_x. This fast, robust model has potential applications in model-based real-time control strategies seeking to reduce feed gas NO_x emissions from diesel engines.     

The fate of char-N at pulverized coal conditions

The fate of char-N (nitrogen removed from the coal matrix during char oxidation) has been widely studied at fluidized bed conditions. This work extends the study of char-N to pulverized coal conditions. Coal chars from five parent coals were prepared and burned in a laboratory-scale pulverized coal combustor in experiments designed to identify the parameters controlling the fate of char-N. The chars were burned with natural gas (to simulate volatiles combustion) in both air and in a nitrogen-free oxidant composed of Ar, CO₂, and O₂. In some experiments, the char flames were doped with various levels of NO or NH₃ to simulate formation of NO_x from volatile-N (nitrogen removed during coal devolatilization). The conversion of char-N to NO_x in chars burned in the nitrogen-free oxidant was 50-60% for lignites and 40-50% for bituminous coals. In char flames doped with NO_x, the apparent conversion of char-N to NO_x (computed using the NO_x measurements made before and after the addition of char to the system) decreased significantly as the level of NO_x doping increased. With 900 ppm NO_x present before the addition of char, apparent conversion of char-N to NO_x was close to 0% for most chars. While there is no clear correlation between nitrogen content of the char and char-N to NO_x conversion at any level of NO_x in the flame, the degree of char burnout within a given family of chars does play a role. Increasing the concentration of O₂ in the system in both air and nitrogen-free oxidant experiments increased the conversion of char-N to NO_x. The effects of temperature on NO_x emissions were different at low (0 ppm) and high (900 ppm) levels of NO_x present in the flame before char addition.     

Experimental study of the combustion efficiency and formation of NOx in an industrial pulverized coal combustor

With horizontal bias combustion burners, experiments have been carried out on a 670 t h^?1, corner‐fired, pulverized‐coal fired boiler burning bituminous coal. At 200 MWe load, the furnace excess O₂ remains stable. The different horizontal fuel biases are obtained by changing the tilt angle of all the Louvre enrichers' regulating blades. The tilt angles of the blades are 0, 15, 24, 32°; the result is that the enriching ratios of the fuel‐rich primary air increase from 2.2 to 2.6 at No. 2 corner, and from 1.2 to 4.2 at No. 3 corner. The gas temperature increases in the burner region. The application of the horizontal bias combustion burners results in a reduction in NO_x formation from 545.7 mg Nm^?3 (O₂=6%) to 287.9 mg Nm^?3, and a substantial reduction in carbon in ash content from 5.24 to 2.48%. The boiler operated stably at a load of 80 MWe without auxiliary fuel oil. Copyright © 2004 John Wiley & Sons, Ltd.     

NOx control in large-scale power plant boilers through superfine pulverized coal technology

Superfine pulverized coal technology can effectively reduce NO _x emission in coal-fired power plant boilers. It can also economize the cost of the power plant and improve the use of the ash in the flue gas. Superfine pulverized coal technology, which will be widely used in China, includes common superfine pulverized coal technology and superfine pulverized coal reburning technology. The use of superfine pulverized coal instead of common coal in large-scale power plants will not only reduce more than 30% of NO _x emission but also improve the thermal efficiency of the boiler. __________ Translated from Journal of Shanghai University of Electric Power, 2006, 22(4): 333–337 [译自: 上海电力学院学报]     

The influence of swirl burner structure on the gas/particle flow characteristics

Improvements were made to a low-NO_x axial swirl burner (LNASB), aimed at mitigating slagging in a 600-MWe boiler burning bituminous coal. The new design is referred to as improved low-NO_x axial swirl burner (ILNASB). This paper describes investigations of the influence of swirl burner structure on the gas/particle flow characteristics using a three-dimensional particle-dynamics anemometer. In comparing results from both ILNASB and LNASB, a central recirculation zone is seen to form in the region x/d = 0.1–0.3 within the ILNASB. This zone had shifted from the region between primary and secondary air in LNASB to a region between inner and outer secondary air. In the vicinity of the burner outlet, particle volume flux is reduced significantly in the central recirculation zone. In contrast, this flux is high near the central axis in ILNASB, thus concentrating a great fraction of pulverized coal near the central axis. Form the study, the gas/particle flow characteristics of the ILNASB show that the improved burner has the ability to ease slagging and reduce NO_x emissions.     

PDA research on a novel pulverized coal combustion technology for a large utility boiler

In this paper, a new technology for a tangential firing pulverized coal boiler, high efficiency and low NO_x combustion technology with multiple air-staged and a large-angle counter flow fuel-rich jet (ACCT for short) is proposed. To verify the characteristics of this technology, experiments of two combustion technologies, ACCT and CFS-1 (Concentric Firing System-1), are carried out under a cold model of a 1025 t/h tangential firing boiler with a PDA (particle dynamics anemometer). The distributions of velocity, particle concentration, particle diameters and the particle volume flux of primary air and secondary air are obtained. The results show that the fuel-rich primary air of ACCT can go deeper into the furnace and mix with the main flow better, which means that the counter flow of fuel-rich jets in ACCT can realize stable combustion, low NO_x emission and slagging prevention.     

NOx emissions and thermal efficiencies of small scale biomass-fuelled combustion plant with reference to process industries in a developing country

Solid biomass materials are an important industrial fuel in many developing countries and also show good potential for usage in Europe within a future mix of renewable energy resources. The sustainable use of wood fuels for combustion relies on operation of plant with acceptable thermal efficiency. There is a clear link between plant efficiency and environmental impacts due to air pollution and deforestation. To supplement a somewhat sparse literature on thermal efficiencies and nitrogen oxide emissions from biomass-fuelled plants in developing countries, this paper presents results for tests carried out on 14 combustion units obtained during field trials in Sri Lanka. The plants tested comprised steam boilers and process air heaters. Biomass fuels included: rubber-wood, fuelwood from natural forests; coconut shells; rice husks; and sugar cane bagasse. Average NO_x (NO and NO₂) emissions for the plants were found to be 47 gNO₂ GJ⁻¹ with 18% conversion of fuel nitrogen. The former value is the range of NO_x emission values quoted for combustion of coal in grate-fired systems; some oil-fired systems and systems operating on natural gas, but is less than the emission levels for the combustion of pulverized fuel and heavy fuel oil. This value is significantly within current European standards for NO_x emission from large combustion plants. Average thermal efficiency of the plants was found to be 50%. Observations made on operational practices demonstrated that there is considerable scope for the improvement of this thermal efficiency value by plant supervisor training, drying of fuelwood and the use of simple instruments for monitoring plant performance.